U.S. patent application number 09/804520 was filed with the patent office on 2002-04-25 for bit allocation method and apparatus therefor.
Invention is credited to Hasegawa, Kazutomo.
Application Number | 20020048334 09/804520 |
Document ID | / |
Family ID | 18797569 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020048334 |
Kind Code |
A1 |
Hasegawa, Kazutomo |
April 25, 2002 |
Bit allocation method and apparatus therefor
Abstract
When the number of transmit bits and gain allocated to each
carrier of a multicarrier transmission system are decided, the S/N
ratio of each carrier is found and a number of transmit bits is
allocated to each of the carriers based upon the respective S/N
ratios thereof. Next, the gains of carriers for which the number of
allocated bits is equal to a maximum limit number are decreased and
the gains of prescribed carriers other than these carriers are
increased. Control is performed in such a manner that the sum total
of gain increases and sum total of gain decreases will be equal, as
a result of which the total of number of transmit bits allocated to
the carriers is increased.
Inventors: |
Hasegawa, Kazutomo;
(kawasaki, JP) |
Correspondence
Address: |
ROSENMAN & COLIN LLP
575 MADISON AVENUE
NEW YORK
NY
10022-2585
US
|
Family ID: |
18797569 |
Appl. No.: |
09/804520 |
Filed: |
March 12, 2001 |
Current U.S.
Class: |
375/360 |
Current CPC
Class: |
H04L 27/2614 20130101;
H04L 5/0044 20130101; H04L 27/2608 20130101; H04L 5/023
20130101 |
Class at
Publication: |
375/360 |
International
Class: |
H04L 027/28 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 19, 2000 |
JP |
2000-319043 |
Claims
What is claimed is:
1. A bit allocation method for deciding number of transmit bits and
gain allocated to each carrier in multicarrier transmission,
comprising the steps of: measuring S/N ratio of each carrier and
allocating a number of transmit bits to each carrier based upon the
S/N ratio; subsequently decreasing the gains of carriers for which
the number of allocated bits is equal to a maximum limit number and
increasing the gains of prescribed carriers other than these
carriers; and performing control in such a manner that the sum
total of gain increases and sum total of gain decreases will be
equal, wherein the total of number of transmit bits allocated to
the carriers is increased.
2. The method according to claim 1, wherein a carrier for which
gain is increased is a carrier for which the number of allocated
bits is large.
3. A bit allocation method for deciding number of transmit bits and
gain allocated to each carrier in multicarrier transmission,
comprising the steps of: measuring S/N ratio of each carrier and
allocating a number of transmit bits to each carrier based upon the
S/N ratio; subsequently increasing the gains of carriers, among
carriers to which bits have not been allocated, for which there is
a high likelihood that a bit will be allocated anew if the gains
thereof are increased, and decreasing the gains of prescribed
carriers other than these carriers; and performing control in such
a manner that the sum total of gain increases and sum total of gain
decreases will be equal, wherein the total number of transmit bits
allocated to the carriers is increased.
4. The method according to claim 3, wherein a carrier for which
gain is decreased is a carrier for which the number of allocated
bits is small but other than two.
5. A bit allocation method for deciding number of transmit bits and
gain allocated to each carrier in multicarrier transmission,
comprising the steps of: measuring S/N ratio of each carrier and
allocating a number of transmit bits to each carrier based upon the
S/N ratio; subsequently decreasing the gains of carriers, among
carriers to which bits have not been allocated, for which there is
little likelihood that a bit will be allocated anew even if the
gains thereof are increased, and increasing the gains of prescribed
carriers other than these carriers; and performing control in such
a manner that the sum total of gain increases and sum total of gain
decreases will be equal, wherein the total number of transmit bits
allocated to the carriers is increased.
6. The method according to claim 5, wherein a carrier for which
gain is increased is a prescribed carrier other than a carrier for
which the number of allocated bits is equal to the maximum limit
number.
7. A bit allocation apparatus for deciding number of transmit bits
and gain allocated to each carrier in multicarrier transmission,
comprising: an S/N ratio measurement unit for measuring S/N ratio
of each carrier; a control unit for allocating a number of transmit
bits to each carrier based upon the S/N ratio, subsequently
decreasing the gains of carriers for which the number of allocated
bits is equal to a maximum limit number, increasing the gains of
prescribed carriers other than these carriers, and performing
control in such a manner that the sum total of gain increases and
sum total of gain decreases will be equal, thereby deciding the
number of bits and gain allocated to each carrier; an allocation
table for storing number of bits and gain that have been allocated
to each carrier; a transmitting unit for transmitting content of
said allocation table to the side of a communicating party; and a
setting unit for setting the number of allocated bits and the gain
of each carrier in a receiving unit which receives and demodulates
data that is transmitted from the communicating party.
8. A bit allocation apparatus for deciding number of transmit bits
and gain allocated to each carrier in multicarrier transmission,
comprising: an S/N ratio measurement unit for measuring S/N ratio
of each carrier; a control unit for allocating a number of transmit
bits to each carrier based upon the S/N ratio, subsequently
increasing the gains of carriers, among carriers to which bits have
not been allocated, for which there is a high likelihood that a bit
will be allocated anew if the gains thereof are increased,
decreasing the gains of prescribed carriers other than these
carriers, and performing control in such a manner that the sum
total of gain increases and sum total of gain decreases will be
equal, thereby deciding the number of bits and gain allocated to
each carrier; an allocation table for storing number of bits and
gain that have been allocated to each carrier; a transmitting unit
for transmitting content of said allocation table to the side of a
communicating party; and a setting unit for setting the number of
allocated bits and the gain of each carrier in a receiving unit
which receives and demodulates data that is transmitted from the
communicating party.
9. A bit allocation apparatus for deciding number of transmit bits
and gain allocated to each carrier in multicarrier transmission,
comprising: an S/N ratio measurement unit for measuring S/N ratio
of each carrier; a control unit for allocating a number of transmit
bits to each carrier based upon the S/N ratio, subsequently
decreasing the gains of carriers, among carriers to which bits have
not been allocated, for which there is little likelihood that a bit
will be allocated anew if the gains thereof are increased,
increasing the gains of prescribed carriers other than these
carriers, and performing control in such a manner that the sum
total of gain decreases and sum total of gain increases will be
equal, thereby deciding the number of bits and gain allocated to
each carrier; an allocation table for storing number of bits and
gain that have been allocated to each carrier; a transmitting unit
for transmitting content of said allocation table to the side of a
communicating party; and a setting unit for setting the number of
allocated bits and the gain of each carrier in a receiving unit
which receives and demodulates data that is transmitted from the
communicating party.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a bit allocation method and
apparatus and, more particularly, to a bit allocation method and
apparatus for deciding the number of bits and gain to be allocated
to each carrier in multicarrier transmission.
[0002] Multimedia services such as the Internet have become
widespread throughout society inclusive of the ordinary home and
there is increasing demand for early provision of economical,
highly reliable digital subscriber line transmission systems and
apparatus for utilizing these services.
[0003] Enormous expenditures of money and time are required to lay
new communication lines. For this reason, a digital subscriber line
transmission system in which existing communication lines are
utilized to communicate data at high speed. A known example of a
technique for providing a digital subscriber line transmission
system is xDSL (Digital Subscriber Line). xDSL is a transmission
scheme that utilizes a telephone line and is one
modulation/demodulation technique. xDSL is broadly divided into
symmetric xDSL and asymmetric xDSL. In symmetric xDSL, the upstream
transmission rate from the subscriber residence (referred to as the
"subscriber.cndot.side" below) to the accommodating office
(referred to as the "office side" below) and the downstream
transmission rate from the office side to the subscriber side are
symmetrical; in asymmetric xDSL, the upstream and downstream
transmission rates are asymmetrical. An example of asymmetric xDSL
is ADSL (Asymmetric DSL), and examples of symmetric xDSL are HDSL
(High-bit-rate DSL) and SHDSL (Single-pair High-bit-rate DSL). With
ADSL, downstream transmission rates are on the order of several
Mbps and upstream transmission rates are on the order of several
hundred kbps. DMT (Discrete Multiple Tone) modulation is
standardized as the modulation scheme by ITU-T recommendations.
[0004] DMT Modulation
[0005] DMT modulation will be described with regard to
modulation/demodulation in the downstream direction from the office
side to the subscriber side.
[0006] With DMT modulation, as shown in FIG. 12, a frequency band
of 1.104 MHz is divided into N-number (255) of multicarriers
#1.about.#255 at intervals of .DELTA.f (=4.3125 KHz). In training
carried out before communication, the S/N ratios of the respective
carriers #1.about.#255 are measured and it is decided, depending
upon the S/N ratios, with which modulation method among 4-QAM,
16-QAM, 64-QAM, 128-QAM . . . modulation methods data is to be
transmitted over each carrier. For example, 4-QAM is assigned to a
carrier having a small S/N ratio and 16-QAM, 64-QAM, 128-QAM . . .
are assigned successively as the S/N ratio increases. It should be
noted that 4-QAM is a modulation scheme in which two bits are
transmitted at a time, 16-QAM a modulation scheme in which four
bits are transmitted at a time, 64-QAM a modulation scheme in which
six bits are transmitted at a time, and 128-QAM a modulation scheme
in which seven bits are transmitted at a time. Among schemes in
which signals are transmitted simultaneously in upstream and
downstream directions, a frequency-division transmission scheme
uses carriers #1.about.#31 of the 255 carriers for the upstream
direction from the subscriber side to the office side, and uses
carriers #33.about.#255 for the downstream direction from the
office side to the subscriber side.
[0007] FIG. 13 is a diagram useful in describing 16-QAM. Here a
serial/parallel converter (S/P converter) 1 stores transmit data,
which enters as a bit serial, in a buffer successively four bits at
a time and outputs four bits as 2-bit parallel data
(a.sub.i,b.sub.i), (a.sub.i+1,b.sub.i+1). A first binary/quaternary
converter 2 converts the parallel data (a.sub.i,b.sub.i) to four
values (-3, -1, +1, +3), and a second binary/quaternary converter 3
converts the parallel data (a.sub.i+1,b.sub.i+1) to four values
(-3, -1, +1, +3). A carrier generator 4 generates a cosine wave cos
(.omega..sub.ct) of frequency f.sub.c (.omega..sub.c=2.pi.f.sub.c),
and a phase shifter 5 shifts the phase of the cosine wave by
90.degree. to output a sine wave sin (.omega..sub.ct). An AM
modulator 6 multiplies the output of the first binary/quaternary
converter 2 by the sine wave sin (.omega..sub.ct), and an AM
modulator 7 multiplies the output of the second binary/quaternary
converter 3 by the cosine wave cos (.omega..sub.ct). An adder 8
combines the outputs of the AM modulators 6 and 7 and outputs the
combined signal. By executing the operation described above, the
16-QAM modulator outputs signals having the illustrated
two-dimensional signal point placement (constellation) in
accordance with the combination of parallel data (a.sub.i,b.sub.i),
(a.sub.i+1,b.sub.i+1). For example, if data divided into four bits
at a time is 1001, 0011, 1100, 0110, the 16-QAM modulator outputs
signals (1).fwdarw.(2).fwdarw.(3).fwdarw.(4) in the
constellation.
[0008] FIG. 14 is diagram useful in describing the principle of DMT
modulation. Bit-serial transmit data enters an S/P converter 11.
From this transmit data, a bit sequence that is to be transmitted
within a certain period is stored in an internal buffer of the S/P
converter 11 and subsequently is output to a carrier mapper 12.
Data transmitted within this fixed period is referred to as a
symbol. Since the QAM modulation scheme of each carrier is known,
the carrier mapper 12 divides the one symbol's worth of bit
sequence b.sub.k-number of bits at a time in accordance with the
QAM modulation scheme of each carrier and inputs the resultant bit
sequence QAM modulator 13i of the particular carrier. As a result,
the total number of output bits per symbol is .SIGMA.b.sub.k (k=1
to N). A frequency multiplexer 14 frequency multiplexes the QAM
signals output from the QAM modulators 13i of the respective
carriers and outputs the multiplexed signal to a transmission line
via a transmission-line drive circuit (not shown).
[0009] Here the frequency multiplexer 14 is provided with an
arithmetic unit for implementing an IFFT (Inverse Fast-Fourier
Transform), whereby transmission based upon DMT modulation is
carried out.
[0010] FIG. 15 is a functional block diagram of a subscriber line
transmission system based upon DMT modulation. Entered transmit
data addressed to a subscriber is stored in an amount conforming to
the time for one symbol (=1/4000 s) in a serial-parallel conversion
buffer (Serial-to-Parallel Buffer) 10. The stored data is divided
into transmit bit counts per carrier decided by training in advance
and saved in a transmit B&G controller 60. The data is then
input to an encoder 20. More specifically, since the QAM modulation
scheme of each carrier is known from training, one symbol's worth
of a bit sequence is divided b.sub.k bits at a time, where the bit
count b.sub.k conforms to the QAM modulation scheme of each
carrier, and the bits are input to the encoder 20. As a result, the
total number of output bits per symbol is .SIGMA.b.sub.k
(k=1.about.N). The encoder 20 converts each input bit sequence
b.sub.k to signal-point data (signal-point data on a constellation
diagram) for performing quadrature amplitude modulation (QAM) and
inputs the converted data to an Inverse Fast-Fourier Transform
(IFFT) unit 30. The IFFT unit 30 applies quadrature amplitude
modulation to each signal point by performing an IFFT operation and
inputs the processed data to a parallel-to-serial conversion buffer
(Parallel-to-Serial Buffer) 40. Here a total of 32 samples, namely
IFFT output samples 480.about.511, are attached to the beginning of
a DMT signal as a cyclic prefix. The parallel-to-serial conversion
buffer 40 inputs 512+32 items of sample data to a D/A converter 50
successively in serial fashion. The D/A converter 50 converts the
input digital data to an analog signal at a sampling frequency of
2.208 MHz and sends the analog signal to the subscriber side via a
metallic line 70.
[0011] On the subscriber side, an A/D converter 80 converts the
input analog signal to a 2.208-MHz digital signal and inputs the
digital signal to a time domain equalizer (TEQ) 90. The latter
applies processing to the input digital data in such a manner that
inter-symbol interference (ISI) will fall within the cyclic prefix
of 32 symbols, and inputs the processed data to a
serial-to-parallel conversion buffer 100. The latter stores one DMT
symbol's worth of data and subsequently removes the cyclic prefix
and inputs one DMT symbol's worth of data to a fast-Fourier
transform (FFT) unit 110 simultaneously in parallel fashion. The
FFT unit 110 implements a fast-Fourier transform and generates
(demodulates) 255 signal points. A frequency domain equalizer (FEQ)
120 subjects the demodulated 255 items of signal-point data to
inter-channel interference (ICI) compensation. A decoder 130
decodes the 255 items of signal-point data in accordance with a
receive B&G controller 150, which has values identical with
those of the transmit B&G controller 60, and stores the data
obtained by decoding in a parallel-to-serial conversion buffer 140.
The data is subsequently read out of this buffer in the form of a
bit serial. This data constitutes the receive data.
[0012] The details of the above-described multicarrier transmission
system are disclosed in John A. C. Bingham, "Multicarrier
Modulation for Data Transmission: An Idea Whose Time Has Come",
IEEE Communications Magazine, Volume 28, Number 5, pp. 5.about.14,
May 1990.
[0013] Setting of Allocated Bits
[0014] The number of bits allocated to each carrier is decided on
the receiving side. Specifically, the number of allocated bits for
an upstream signal is decided on the office side and the number of
allocated bits for a downstream signal is decided on the subscriber
side. When training is performed, ADSL units on the office and
subscriber sides decide the allocated bits in accordance with a
protocol referred to as B&G (bit and gain).
[0015] FIG. 16 is a diagram useful in describing an overview of the
B&G protocol for the downstream direction. (1) When training is
performed, the ADSL units recognize each other and then the ADSL
unit ATU-C on the office side sends several frequency signals to
the opposing ADSL unit ATU-R on the subscriber side. (2) The ADSL
unit ATU-R on the subscriber side calculates the S/N ratio of each
carrier. (3) Next, the ADSL unit ATU-R on the subscriber side
decides the allocated bits of each carrier based upon the
calculated S/N ratio of each carrier and reports the allocated bits
and transmission level (gain) to the ADSL unit ATU-C on the office
side. (4) The ADSL unit ATU-C on the office side performs DMT
modulation based upon the reported allocated bits and
transmission-level information and transmits the resultant
data.
[0016] An example of a method of setting an allocation table
indicative of the number of allocated bits and gain of each carrier
is disclosed in Peter S. Chow, John M. Cioffi, "Method and
apparatus for adaptive, variable bandwidth, high-speed data
transmission of a multicarrier signal over digital subscriber
lines", U.S. Pat. No. 5,479,447. The theory on which this is based
will now be described in simple terms.
[0017] FIG. 17 illustrates the relationship between an S/N-ratio
curve (S/N curve), which indicates the ratio of the size of a
receive signal for each frequency to the magnitude of noise
inflicted upon this receive signal, and number of bits allocated to
each carrier. If the S/N ratio at the time of frequency
n.multidot.f.sub.d is SNR.sub.n for a carrier #n whose frequency is
n.multidot.f.sub.d, the optimum number b.sub.n of bits to allocated
to the carrier #n is calculated in accordance with the following
equation:
b.sub.n=log.sub.2(1+SNR.sub.n/.GAMMA.) (1)
[0018] where n represents a positive integer that is equal to or
less than N and f.sub.d represents carrier spacing. In the example
of FIG. 12, N=255 and f.sub.d=4.3125 kHz hold. Further, .GAMMA.
represents SNR gap.
[0019] The optimum number b.sub.n of bits in many cases is a
decimal number, as indicated by the dashed line in FIG. 17. For
example, if the S/N ratio SNR6 of carrier #6 (=frequency
6.multidot.f.sub.d) is inserted into Equation (1), the optimum bit
count b.sub.6 is about 4.2, which is a decimal. However, the number
of bits actually allocated to each carrier can take on only an
integral value. Accordingly, the values indicated by the solid line
obtained by discarding the decimal part of the calculated values
become the numbers of bits actually allocated to the carriers. In
the above-mentioned example, four bits, which is the number
obtained by discarding the decimal part of 4.2, are allocated to
carrier #6. Similarly, bits are allocated to the other carriers
upon discarding decimals so that the relationship between carriers
and numbers of allocated bits becomes as shown in FIG. 18. It
should be noted that when the optimum bit count calculated in
accordance with Equation (1) is equal to or greater than 1.0 and
less than 2.0, at present not even one bit is allocated, as
indicated by FIG. 17. If the maximum limit on allocated bits is
five, then, even in a case where the optimum allocated bit count
indicated by the dashed line in FIG. 17 is six or greater, the
number of bits allocated is limited to five, as shown by the solid
line in FIG. 17.
[0020] In the example set forth above, the number of bits actually
allocated to each carrier is decided upon discarding the decimal
part. However, a method that utilizes a calculated bit count
without discarding the decimal part also is available. For example,
in a case where the optimum bit count b.sub.6 is calculated to be
about 4.2 in accordance with Equation (1), this method finds a
power .DELTA.x necessary to obtain a total bit count of five by
allocating 0.8 bits, stores the power .DELTA.x and the allocated
bit count (five in this example), which is based upon the power
increase, in the receive B&G controller 150 and simultaneously
notifies the transmit B&G controller 60 on the transmitting
side. It should be noted that if the original power is made 1 by
normalization, then changing power by .+-..DELTA.x is the same as
making the additional gain .+-..DELTA.x.
[0021] In actuality, when training is performed prior to data
communication, the content of an updated receive allocation table
is reported from the receiving side to the transmitting side and
the bit allocation table of the transmit B&G controller on the
transmitting side is updated so as to make its content identical
with that of the bit allocation table on the receiving side. Data
is transmitted on the transmitting side based upon the updated
information in the bit allocation table. In this example,
transmission with regard to carrier #6 is performed using five bits
as the number of allocated bits and .DELTA.x as the additional
gain. In a case where the additional gain of a carrier is varied
and made .DELTA.x, the additional gain of another carrier is made
-.DELTA.x so that the total additional gain of all carriers will be
zero. This is because it is necessary to make transmission power
uniform for use in linear characteristic components.
[0022] With conventional bit allocation, as described above, how
much additional gain is necessary to increase the allocation of one
bit is calculated from the S/N ratio at the output of the FEQ 120,
and the number of allocated bits is calculated in accordance with
Equation (1) taking this additional gain into consideration. Though
this method does make it possible to obtain a favorable bit
allocation by calculating the optimum additional gain, the process
through which the optimum additional gain is calculated is
complicated and the optimum additional gain and bit allocation
cannot be acquired in a short period of time. Thus there is sought
a bit allocation method and apparatus through which the optimum
additional gain and bit allocation can be calculated in a short
time by simplifying the process for finding the additional
gain.
SUMMARY OF THE INVENTION
[0023] Accordingly, an object of the present invention make it
possible to calculate optimum additional gain and bit allocation in
a short time.
[0024] Another object of the present invention is to make it
possible to increase the total number of transmitted bits allocated
to the carriers without raising transmission power, thereby
improving the transmission capability of a multicarrier
transmission apparatus.
[0025] A first bit allocation method according to the present
invention (1) measures the SIN ratio of each carrier and allocates
a number of transmit bits to each carrier based upon the S/N ratio;
(2) subsequently decreases the gains of carriers for which the
number of allocated bits is equal to a maximum limit number and
increases the gains of prescribed carriers other than these
carriers; and (3) performs control in such a manner that the sum
total of gain increases and the sum total of gain decreases will be
equal, wherein the total of number of transmit bits allocated to
the carriers is increased.
[0026] Though the number of bits allocated to each carrier is
decided in dependence upon the S/N ratio of the carrier, the number
of allocated bits cannot be increased beyond a maximum limit number
regardless of how good the S/N ratio becomes. In other words, a
carrier to which the maximum limit number has been allocated has
some surplus in terms of gain. Hence, if the surplus gain can be
decreased and the gains of other carriers increased, then the total
number of allocated bits can be increased. The present invention
takes note of this and is capable of increasing the total of number
of transmit bits allocated to the carriers without raising
transmission power, thereby making it possible to improve the
transmission capability of a multicarrier transmission
apparatus.
[0027] In this case, a carrier for which gain is increased is made
a carrier for which the number of allocated bits is large. This is
because the number of bits can be increased by one with a power
that is lower for a carrier for which the number of allocated bits
is large than for a carrier for which the number of allocated bits
is small.
[0028] A second bit allocation method according to the present
invention (1) measures the S/N ratio of each carrier and allocates
a number of transmit bits to each carrier based upon the S/N ratio;
(2) subsequently increases the gains of carriers, among carriers to
which bits have not been allocated, for which there is a high
likelihood that bits will be allocated anew if the gains thereof
are increased, and decreases the gains of prescribed carriers other
than these carriers; and (3) performs control in such a manner that
the sum total of gain increases and the sum total of gain decreases
will be equal, wherein the total number of transmit bits allocated
to the carriers is increased.
[0029] Even if a carrier is one to which even one transmit bit has
not been allocated due to an inadequate S/N ratio, it becomes
possible to transmit two bits at a stroke if gain is increased
(because 1-bit QAM modulation does not exist). Accordingly, if the
gain of a carrier to which transmit bits have not been allocated is
increased and the gains of other carriers are decreased, the total
number of allocated bits can be increased. The present invention
takes note of this and is capable of increasing the total number
transmit bits allocated to the carriers without raising
transmission power, thereby making it possible to improve the
transmission capability of a multicarrier transmission
apparatus.
[0030] In this case, a carrier for which gain is decreased is made
a carrier for which the number of allocated bits is small but other
than two. This is because there are cases where if the gain of a
carrier for which the number of allocated bits is two is decreased,
the number of allocated bits changes from two to zero owing to the
decrease in gain. Further, the number of allocated bits can be
decreased by one with a power that is larger for a carrier for
which the number of allocated bits is small than for a carrier for
which the number of allocated bits is large.
[0031] A third bit allocation method according to the present
invention (1) measures the S/N ratio of each carrier and allocates
a number of transmit bits to each carrier based upon the S/N ratio;
(2) subsequently decreases the gains of carriers, among carriers to
which bits have not been allocated, for which there is little
likelihood that bits will be allocated anew even if the gains
thereof are increased, and increases the gains of prescribed
carriers other than these carriers; and (3) performs control in
such a manner that the sum total of gain increases and the sum
total of gain decreases will be equal, wherein the total number of
transmit bits allocated to the carriers is increased.
[0032] A carrier to which a transmit bit has not been allocated due
to an inadequate S/N ratio and which is not likely to be allocated
a bit anew even if gain is increased is a carrier that is useless.
Accordingly, if a reduction is made in the gains of carriers, among
carriers to which bits have not been allocated, for which there is
little likelihood that bits will be allocated anew even if the
gains thereof are increased, and if the gains of prescribed
carriers other than these carriers are increased, then the total
number of allocated bits can be increased. The present invention
takes note of this and is capable of increasing the total number of
transmit bits allocated to the carriers without raising
transmission power, thereby making it possible to improve the
transmission capability of a multicarrier transmission
apparatus.
[0033] In this case, a carrier for which gain is increased is a
prescribed carrier other than a carrier for which the number of
allocated bits is equal to the maximum limit number. This is
because even if the gain of a carrier for which the number of
allocated bits is equal to the maximum limit number is increased,
the number of allocated bits does not increase.
[0034] Other features and advantages of the present invention will
be apparent from the following description taken in conjunction
with the accompanying drawings, in which like reference characters
designate the same or similar parts throughout the figures
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a block diagram illustrating the configuration of
a subscriber line transmission system based upon DMT modulation
according to the present invention;
[0036] FIG. 2 shows an algorithm of a first bit allocation method
according to the present invention;
[0037] FIG. 3 is a diagram useful in describing the first bit
allocation method according to the present invention;
[0038] FIG. 4 is a table useful in describing number of allocated
bits according to the first bit allocation method of the present
invention;
[0039] FIG. 5 is a flowchart of bit allocation processing at the
time of training;
[0040] FIG. 6 shows an algorithm of a second bit allocation method
according to the present invention;
[0041] FIG. 7 is a diagram useful in describing the second bit
allocation method according to the present invention;
[0042] FIG. 8 is a table useful in describing number of allocated
bits according to the second bit allocation method of the present
invention;
[0043] FIG. 9 shows an algorithm of a third bit allocation method
according to the present invention;
[0044] FIG. 10 is a diagram useful in describing the third bit
allocation method according to the present invention;
[0045] FIG. 11 is a table useful in describing number of allocated
bits according to the third bit allocation method of the present
invention;
[0046] FIG. 12 is a diagram useful in describing a DMT transmission
spectrum according to the prior art;
[0047] FIG. 13 is a diagram useful in describing 16-QAM according
to the prior art;
[0048] FIG. 14 is a diagram useful in describing the principle of
DMT modulation according to the prior art;
[0049] FIG. 15 is a functional diagram of a subscriber transmission
system which relies upon DMT modulation according to the prior
art;
[0050] FIG. 16 is a diagram useful in describing a B&G protocol
according to the prior art;
[0051] FIG. 17 is a diagram useful in describing the relationship
among S/N ratio, optimum number of bits and actual allocated number
of bits according to the prior art; and
[0052] FIG. 18 is a table showing the relationship between carriers
and numbers of allocated bits according to the prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0053] (A) Configuration
[0054] FIG. 1 is a block diagram illustrating the configuration of
a subscriber line transmission system based upon DMT modulation
according to the present invention. An xDSL unit 200 on the office
side and an xDSL on the subscriber side are connected for
bidirectional communication by a telephone line (metallic line)
400.
[0055] The office-side xDSL unit 200 and the subscriber-side xDSL
unit 300 have transmitting sections 210, 310, respectively, each of
which has components identical with the components 10 to 50 on the
transmitting side shown in FIG. 15, and receiving sections 220,
320, respectively, each of which has components identical with the
components 80 to 140 on the receiving side shown in FIG. 15. The
office-side xDSL unit 200 further includes a transmit B&G
controller 230 corresponding to the transmit B&G controller 60
of FIG. 15, and the subscriber-side xDSL unit 300 further includes
a receive B&G controller 330 corresponding to the receive
B&G controller 150 of FIG. 15.
[0056] Only components for implementing a B&G protocol in the
downstream direction are illustrated for the transmit B&G
controller 230 and receive B&G controller 330; components for
implementing a B&G protocol in the upstream direction are not
shown. However, the components for implementing the B&G
protocol in the upstream direction are similar to those for
implementing the B&G protocol in the downstream direction and
portions that can be shared are shared.
[0057] The transmit B&G controller 230 has a bit/gain
allocation unit 231 for storing a bit allocation table sent from
the receive B&G controller 330 and allocating a bit count B and
gain G to each carrier; an allocation-table storage unit 232 for
storing a bit allocation table; and a bit/gain setting unit 233 for
setting the allocated bit count B and gain G in the serial/parallel
converter 10 and encoder 20 on a per-carrier basis.
[0058] The receive B&G controller 330 includes an S/N
measurement unit 331 for measuring the S/N ratio of each carrier
based upon the output of the FEQ 120; a bit/gain allocation unit
332 for allocating bit/gain to each carrier based upon the S/N
ratio of each carrier; an allocation-table storage unit 333 for
storing a correspondence table (bit allocation table) indicating
the correspondence between carriers and bit/gain allocated thereto;
and an allocated-bit/gain setting unit 334 for setting an allocated
bit count B and gain G in the decoder 130 and parallel/serial
converter 140 of the receiving section 320. It should be noted that
the bit/gain allocation unit 332 sends the generated bit allocation
table to the transmit B&G controller 230 via the transmitting
section 310, metallic line 400 and receiving section 220 in the
order mentioned.
[0059] (B) First Allocation Method
[0060] The number of bits allocated to each carrier in multicarrier
transmission is decided in accordance with the S/N ratio. However,
the number of allocated bits cannot be increased beyond a maximum
limit number regardless of how good the S/N ratio becomes. In other
words, a carrier to which the maximum limit number has been
allocated has gain to spare. Accordingly, if surplus gain of this
carrier is decreased and the gains of other carriers increased,
then the total number of allocated bits can be increased. This idea
forms the basis of the present invention.
[0061] FIG. 2 shows the algorithm of a first bit allocation method
implemented by the receive B&G controller 330 according to the
present invention, and FIG. 3 is a diagram useful in describing the
first bit allocation method.
[0062] First, bit allocation based upon a well-known method is
carried out (step 501) and then the total bit count b_base, namely
the total number of bits, that has been allocated to all carriers
is calculated (step 502). With reference to FIG. 3, the bit/gain
allocation unit 332 decides an optimum allocated bit count bn (see
the dashed line in FIG. 3) in accordance with Equation (1) based
upon the S/N ratio curve obtained by the S/N measurement unit 331.
The solid line, which is obtained by discarding the decimal part of
the optimum allocated bit count indicated by the dashed line,
becomes the number of bits allocated by the already known bit
allocation method. It should be noted that if the maximum limit on
number of allocated bits is five, the number of bits actually
allocated is restricted to five even in a case where optimum number
of allocated bits indicated by the dashed line is 6.0 or greater.
In accordance with the already known bit allocation method set
forth above, bits are allocated to each of the carriers #1 to #12
and b_base is equal to 35, as illustrated in FIGS. 17 and 18
indicative of the prior art.
[0063] When the calculation of total bit count b_base at step 502
is completed, the bit/gain allocation unit 332 checks the range
#n_max.sub.--1 to #n_max_k of carriers to which bits of the maximum
limit number (=5) have been allocated (step 503). In FIG. 3, the
range of carriers to which the maximum limit number of bits have
been allocated is #2 to #5, and k=4 holds.
[0064] Next, the powers of the carriers #n_max_1 to #n_max_k
(carriers #2 to #5) are decreased by .DELTA.x (for a total power
reduction of k.multidot..DELTA.x) (step 504), and the powers of the
plurality of carriers having large numbers of allocated bits, which
carriers are other than the carriers #n_max.sub.--1 to #n_max_k
(carriers #2 to #5), are increased up to a total power of
k.multidot..DELTA.x (step 505). The reason for adopting carriers
having large numbers of allocated bits as the carriers for which
power is increased is that when the power for increasing the number
of allocated bits by one bit is taken into account, the increase of
one bit can be achieved with less power in the case of carriers for
which the number of allocated bits is large. It should be noted
that if the original power is made 1 by normalization, then
increasing or decreasing power by .+-..DELTA.x is equivalent to
changing gain by .+-..DELTA.x.
[0065] After the power of each carrier is increased or decreased,
the number of allocated bits of each carrier is calculated in
accordance with Equation (1) and the sum total b_sum of numbers of
bits obtained when the decimal parts of the bit counts are
discarded is calculated (step 506). This is illustrated by the
combinations of arrows and black dots in FIG. 3. The powers of
carriers #2 to #5 are decreased by .DELTA.x each (for a total
decrease of 4.multidot..DELTA.x), and the powers of carriers #1,
#6, #7, #8 are increased correspondingly by .DELTA.x each. In the
case of FIG. 3, the number of bits allocated to each carrier
becomes as shown in FIG. 4, and b_sum is equal to 38. It should be
noted that an arrangement may be adopted in which the powers of any
three carriers are increased for a total increase of
4.multidot..DELTA.x rather than increasing the powers of the
carriers #1, #6, #7, #8 by .DELTA.x each.
[0066] Next, the total bit count b_base obtained by the known
method and the total allocated bit count b_sum after the power
increase/decrease are compared in terms of magnitude (step 507). If
b_sum>b_base holds or k=0 holds, first bit allocation processing
of the present invention is exited (step 508). That is, the
bit/gain allocation unit 332 creates an allocation table (see FIG.
4), which includes the number B of bits allocated to each carrier
and the additional gain G of each carrier, stores this table in the
allocation-table storage unit 333 and then sends the table to the
office side from the transmitting section 310.
[0067] If it is found that b_sum.ltoreq.b_base holds ("NO" at step
507), k is reduced (step 509) and the processing from step 503
onward is repeated. In the example of FIG. 4, the carriers of
interest are carriers #2 to #5 (k=4) and therefore the number of
carriers of interest may be reduced from the high-frequency end, as
in the manner #3 to #5 (k=3) or from the low-frequency end, as in
the manner #2 to #4 (k=3), by way of example. Then b_sum is
obtained again and processing continues until a "YES" decision is
rendered at step 507. However, if k=0 is found to hold at step 507,
processing is terminated, even if a "YES" decision is not obtained,
and bits are allocated in accordance with bit allocation by the
known method (where the total number of bits is b_base).
Furthermore, even in a case where a "YES" decision is rendered at
step 507 of FIG. 2 and processing would ordinarily be exited, the
number k of carriers of interest may be reduced if desired to
search for the optimum bit allocation.
[0068] Thus, in accordance with the first bit allocation method of
the present invention, the total number of transmit bits allocated
to carriers can be increased without raising total transmission
power, thereby making it possible to improve the transmission
capability of a multicarrier transmission apparatus.
[0069] Modification
[0070] The present invention as set forth above is applied to a
case where the transmission capability of a multicarrier
transmission apparatus is to be improved. However, the invention
can be applied also to bit allocation processing carried out at
training of a subscriber line transmission system. FIG. 5 is a
flowchart of bit allocation processing executed at the time of
training in accordance with the present invention.
[0071] In a subscriber line transmission system that relies upon
DMT modulation, one symbol period is {fraction (1/4000)} s (=250
.mu.s) and the symbol is composed of M bits. Accordingly, it is
necessary to subject M bits to multicarrier transmission in one
symbol period.
[0072] At training time, therefore, bit allocation is performed by
the known allocation method (step 551) and it is determined whether
the allocation of M bits has been achieved (step 552). If M bits
could be allocated ("YES" at step 552), bit allocation processing
is exited. If M bits could not be allocated ("NO" at step 552),
however, then M bits are allocated upon increasing the number of
allocated bits by executing processing from step 502 onward in the
bit allocation algorithm of FIG. 2 (step 553).
[0073] (C) Second Allocation Method
[0074] FIG. 6 shows the algorithm of a second bit allocation method
implemented by the receive B&G controller 330 according to the
present invention, and FIG. 7 is a diagram useful in describing the
second bit allocation method.
[0075] First, bit allocation based upon a well-known method is
carried out (step 601) and then the total bit count b_base, namely
the total number of bits, that has been allocated to all carriers
is calculated (step 602). The example of FIG. 7 is similar to that
of the example of FIG. 3 and the details thereof need not be
described again. The result of this processing is that bits are
allocated to each of the carriers #1 to #12 and b_base becomes
equal to 35, as illustrated in FIG. 17.
[0076] When the calculation of total bit count b_base at step 602
is completed, the range #n_min.sub.--1 to #n_min_k of carriers of
interest to which zero bits are allocated is checked (step
603).
[0077] A carrier of interest to which no bits are allocated is a
carrier, among carriers for which the bit allocation is zero, for
which there is a high likelihood that a bit will be allocated anew
by an increase in power. In actuality, the higher the frequency,
the lower the S/N curve, as illustrated in FIG. 7. This means that
the higher the frequency of the carrier, the greater the
possibility that a bit will not be allocated to the carrier.
Accordingly, there are many cases where several carriers on the
high-frequency side of the carrier of maximum frequency to which
bits have been allocated become the carrier range of interest. In
FIG. 7, bits have been allocated up to carrier #9; hence, carriers
#10 to #11 on the side of higher frequency constitute the range of
carriers of interest. In this case, k=2 holds.
[0078] Next, the powers of the carriers #n_min.sub.--1 to #n_min_k
(carriers #10 to #11) are increased by .DELTA.x (for a total power
increase of k.multidot..DELTA.x) (step 604), and the powers of
carriers having numbers of allocated bits that are as small as
possible, which carriers are other than the carriers #n_min.sub.--1
to #n_min_k (carriers #10 to #11), are decreased to a total power
of k.multidot..DELTA.x (step 605). It is assumed here that the
carriers for which power is decreased do not include carriers #8 to
#9 to each of which two bits have been allocated. The reason for
this is that if the power of a carrier for which the number of
allocated bits is two is reduced, the number of allocated bits may
change from two to zero. Accordingly, a carrier having a small
number of allocated bits signifies a carrier to which three bits
have been allocated.
[0079] After the power of each carrier is increased or decreased,
the number of allocated bits of each carrier is calculated in
accordance with Equation (1) and the sum total b_sum of numbers of
bits obtained when the decimal parts of the bit counts are
discarded is calculated (step 606). This is illustrated by the
combinations of arrows and black dots in FIG. 7. The powers of
carriers #10 to #11 are increased by .DELTA.x each (for a total
increase of 2.multidot..DELTA.x), and the powers of carriers #6, #7
are decreased correspondingly by .DELTA.x each. It should be noted
that it will suffice if the total power reduction is
2.multidot..DELTA.x, e.g., the power of either carrier #6 or #7 may
be decreased by 2.multidot..DELTA.x instead of decreasing the
powers of each of carriers #6, #7 by .DELTA.x. In the case of FIG.
7, the number of bits allocated to each carrier becomes as shown in
FIG. 8, and b_sum is equal to 37.
[0080] Next, the total bit count b_base obtained by the known
method and the total allocated bit count b sum after the power
increase/decrease are compared in terms of magnitude (step 607). If
b_sum>b_base holds or k=0 holds, second bit allocation
processing of the present invention is exited (step 608). That is,
the bit/gain allocation unit 332 creates an allocation table, which
includes the number B of bits allocated to each carrier and the
additional gain G of each carrier, stores this table in the
allocation-table storage unit 333 and then sends the table to the
office side from the transmitting section 310.
[0081] If it is found that b_sum.ltoreq.b_base holds ("NO" at step
607), k is reduced (step 609) and then the processing from step 603
onward is repeated. Then b_sum is obtained again and processing
continues until a "YES" decision is rendered at step 607. It should
be noted that even in a case where a "YES" decision is rendered at
step 607 of FIG. 6 and processing would ordinarily be exited, the
number k of carriers of interest may be reduced if desired to
search for the optimum bit allocation.
[0082] Thus, in accordance with the second bit allocation method of
the present invention, the total number of transmit bits allocated
to the carriers can be increased without raising total transmission
power, thereby making it possible to improve the transmission
capability of a multicarrier transmission apparatus.
[0083] The present invention as set forth above is applied to a
case where the transmission capability of a multicarrier
transmission apparatus is to be improved. However, the invention
can be applied also to bit allocation processing (FIG. 5) carried
out at training of a subscriber line transmission system.
[0084] (D) Third Allocation Method
[0085] FIG. 9 shows the algorithm of a third bit allocation method
implemented by the receive B&G controller 330 according to the
present invention, and FIG. 10 is a diagram useful in describing
the third bit allocation method.
[0086] First, bit allocation based upon a well-known method is
carried out (step 701) and then the total bit count b_base, namely
the total number of bits, that has been allocated to all carriers
is calculated (step 702). The example of FIG. 10 is similar to that
of the example of FIG. 3 and the details thereof need not be
described again. The result of this processing is that bits are
allocated to each of the carriers #1 to #12 and b_base becomes
equal to 35, as illustrated in FIG. 17.
[0087] When the calculation of total bit count b_base at step 702
is completed, the range #n_min.sub.--1 to #n_min_k of carriers of
interest to which zero bits are allocated is checked (step
703).
[0088] A carrier of interest to which no bits are allocated is a
carrier, among carriers for which the bit allocation is zero, for
which there is a little likelihood that a bit will be allocated
anew even if power is increased. In actuality, among carriers to
which zero bits are allocated, carriers belonging to the
high-frequency side often constitute the carrier range of interest,
as illustrated in FIG. 10. In FIG. 10, carrier #12 on the
high-frequency side is the carrier of interest among carriers to
which zero bits are allocated. In this case, k=1 holds.
[0089] Next, the powers of the carriers #n_min.sub.--1 to #n_min_k
(carrier #12) are decreased by .DELTA.x (for a total power decrease
of k.multidot..DELTA.x) (step 704), and the powers of carriers
having numbers of allocated bits that are as large as possible,
which carriers are other than the carriers #n_min.sub.--1 to
#n_min_k (carrier #12), are increased up to a total power of
k.multidot..DELTA.x (step 705). It is assumed here that the
carriers for which power is increased do not include carriers for
which the number of allocated bits is equal to the maximum limit
number. The reason for this is that even if the gain of a carrier
for which the number of allocated bits is equal to the maximum
limit number is increased, the number of allocated bits does not
increase. Accordingly, a carrier whose power is increased in one
whose number of allocated bits if four in the case of FIG. 10.
[0090] After the power of each carrier is increased or decreased,
the number of allocated bits of each carrier is calculated in
accordance with Equation (1) and the sum total b_sum of numbers of
bits obtained when the decimal parts of the bit counts are
discarded is calculated (step 706). This is illustrated by the
combinations of arrows and black dots in FIG. 10. The power of
carrier #12 is decreased by .DELTA.x, and the power of carrier #1
is increased correspondingly by .DELTA.x. It should be noted that
instead of increasing the power of carrier #1 by .DELTA.x, power
may be increased by a total of .DELTA.x by splitting the power
increase among two or more carriers. In the case of FIG. 10, the
number of bits allocated to each carrier becomes as shown in FIG.
11, and b_sum is equal to 36.
[0091] Next, the total bit count b_base obtained by the known
method and the total allocated bit count b_sum after the power
increase/decrease are compared in terms of magnitude (step 707). If
b_sum>b_base holds, third bit allocation processing of the
present invention is exited (step 708). That is, the bit/gain
allocation unit 332 creates an allocation table, which includes the
number B of bits allocated to each carrier and the additional gain
G of each carrier, stores this table in the allocation-table
storage unit 333 and then sends the table to the office side from
the transmitting section 310.
[0092] If it is found that b_sum.ltoreq.b_base holds ("NO" at step
707), processing is exited. This is because there is no change in
result even if the range of the number k of carriers of interest is
narrowed.
[0093] Thus, in accordance with the third bit allocation method of
the present invention, the total number of transmit bits allocated
to the carriers can be increased without raising total transmission
power, thereby making it possible to improve the transmission
capability of a multicarrier transmission apparatus.
[0094] The present invention as set forth above is applied to a
case where the transmission capability of a multicarrier
transmission apparatus is to be improved. However, the invention
can be applied also to bit allocation processing (FIG. 5) carried
out at training of a subscriber line transmission system.
[0095] In the case described above, the first to third bit
allocation algorithms of the present invention are used
independently of one another. However, these algorithms can be used
in combination.
[0096] Thus, in accordance with the present invention, (1) the S/N
ratio of each carrier is measured and a number of transmit bits is
allocated to each carrier based upon the S/N ratio; (2)
subsequently the gains of carriers for which the number of
allocated bits is equal to a maximum limit number are decreased and
the gains of prescribed carriers other than these carriers are
increased; and (3) control is exercised in such a manner that the
sum total of gain increases and sum total of gain decreases will be
equal. As a result, the total number of transmit bits allocated to
the carriers can be increased without raising transmission power,
thereby making it possible to improve the transmission capability
of a multicarrier transmission apparatus. In this case, a carrier
for which gain is increased is made a carrier for which the number
of allocated bits is large. The result is that the total number of
allocated bits can be increased.
[0097] Further, in accordance with the present invention, (1) the
S/N ratio of each carrier is measured and a number of transmit bits
is allocated to each carrier based upon the SIN ratio; (2) the
gains of carriers, among carriers to which bits have not been
allocated, for which there is a high likelihood that bits will be
allocated anew if the gains thereof are increased, are increased,
and the gains of prescribed carriers other than these carriers are
decreased; and (3) control is exercised in such a manner that the
sum total of gain increases and sum total of gain decreases will be
equal. As a result, the total number of transmit bits allocated to
the carriers can be increased without raising transmission power,
thereby making it possible to improve the transmission capability
of a multicarrier transmission apparatus. In this case, a carrier
for which gain is decreased is made a carrier for which the number
of allocated bits is small but other than two. The result is that
the total number of allocated bits can be increased.
[0098] Further, in accordance with the present invention, (1) the
S/N ratio of each carrier is measured and a number of transmit bits
is allocated to each carrier based upon the S/N ratio; (2) the
gains of carriers, among carriers to which bits have not been
allocated, for which there is little likelihood that bits will be
allocated anew even if the gains thereof are increased, are
decreased, and the gains of prescribed carriers other than these
carriers are increased; and (3) control is exercised in such a
manner that the sum total of gain increases and sum total of gain
decreases will be equal. As a result, the total number of transmit
bits allocated to the carriers can be increased without raising
transmission power, thereby making it possible to improve the
transmission capability of a multicarrier transmission apparatus.
In this case, a carrier for which gain is increased is a carrier
other than a carrier for which the number of allocated bits is
equal to the maximum limit number. The result is that the total
number of allocated bits can be increased.
[0099] As many apparently widely different embodiments of the
present invention can be made without departing from the spirit and
scope thereof, it is to be understood that the invention is not
limited to the specific embodiments thereof except as defined in
the appended claims.
* * * * *